There use to be such a time (usually the Spring) but now I think that line has become blurred mainly because MH can now be used throughout the year without any discomfort. Unless your buying a trade in from a dealer then they're some bargains to be had
Now at this time of year I would say. then you can get ready for all the events fun holds and shows.
festivals .etc .we bought our first one in march then out in time for easter froze to death but wonderful .the second one bought in september too good to miss.
But was the end of our season so again proper first time out at easter.
I think new first time owners are more likely to buy in Spring as the weather is getting nice.. We used to have a branch of Brownhills just a few hundred yards away and knew the sales staff quiet well. On a sunny weekend it was packed but get a rainy one and the place was dead.
More experienced users would maybe buy anytime of the year, all depending on if they have got their current van sold...
So I guess anytime could be the right time..